feat!: add support of new custom constant completer syntax

[blindFS]

nushell/nushell#16789

A breaking change since param_cmd is now renamed as param_completer.

[blindFS]

CC @Bahex

[Bahex]

@blindFS wow thanks for the quick update (after I added yet more syntax 😔)

I got confused about val_record in param_completer at first, then I saw the test case.

def test [
  x: int@[1 2],
  y: string@{f: [a b]}.f,
  --zero(-z): list<string@$var.cell?>
= [a b],
] {}

Looks good to me!

[fdncred]

Thanks